The present invention relates to a vat paper machine which comprises a cylinder-vat unit having a rotatable cylinder mold, a vat and a headbox connected to the vat for providing a continuous supply of stock to a predetermined level in the vat.
A vat paper machine with a vat section is know through U.S. Pat. No. 4,659,432. The vat section is provided with a platform between the breast roll and the cylinder mold, said platform supporting the outer wire and being provided with slit means to form one or more drainage passages to produce a second web on the outer wire by draining water from the stock through one or more of said drainage passages. The first and second webs are then couched together to form a unitary web which is removed from the cylinder mold. The part of the vat bottom located nearest the nip between outer wire and cylinder mold is provided with a sealing lip arranged to press the outer wire against the cylinder mold. Particularly when it is desired to run this vat paper machine with fully closed platform with the intention to produce a web consisting of a single layer which is then formed on the cylinder mold, it has proved difficult to achieve a perfect seal at the bottom of the vat in the vicinity of said nip so that fibres may be deposited on the outer wire before it joins the web formed on the cylinder mold. The undesired layer of fibres is deposited unevenly on the outer wire so that a paper is produced having a correspondingly uneven grammage. Furthermore, irregular flows may occur in the stock before the nip, due to said undesired drainage through the outer wire in the vicinity of the nip. These flows have an unfavourable influence on the forming of the web on the cylinder mold, in that portions of the web are washed away so that the fibre distribution and grammage of the web becomes uneven. These problems are particularly serious in the manufacture of security paper with water marks, where the wire gauze on the cylinder molds has sections designed therein giving deviating paper thickness. Said sections have recesses which form openings between the outer wire and the cylinder mold, through which the stock can be pressed out so that fibres are deposited on the outer wire. Said openings cannot be closed by said sealing lip since this is permitted only to be pressed against the cylinder mold with a limited pressure so as not to destroy the raised portions of said sections which are to produce the water marks.
SE No. 109 941 describes an apparatus for dewatering a fibre suspension between a rotating screen cylinder and an endless belt of water-impervious material such as rubber. Similar apparatus are described in DE No. 1 072 877 and U.S. Pat No. 2,929,448. U.S. Pat No. 2,974,726 relates to a cylinder-vat unit equipped with an endless liquid-impervious the circumference of the cylinder mold. However, these known apparatus are completely different from the vat paper machine according to the present invention. Said belts are utilized for completely different purposes and none of the specifications either suggests or intimates that they might be suitable for solving the specific problem encountered in the vat paper machine defined in the introduction.